Microtubules are involved in chromosome segregation, cytoplasmic transport, anisometric cell shape, and flagellar motility. We are studying intermediates in in vitro microtubule assembly and disassembly, and several post-translational modifications of tubulin subunits which may modulate assembly or function. These include phosphorylation of beta-tubulin, transformations of tubulin-bound nucleotides, and a novel reaction by which a single tyrosine residue is reversibly linked to alpha-tubulin. We are also studying microtubule associated energy transducing proteins in flagella, from the standpoint of motility and Ca plus 2-activated tactic responses.